Multi-layer circuit board having at least two or more circuit patterns connected

ABSTRACT

A circuit board having stable connection resistance can be obtained . The multi-layer circuit board includes the steps of making through-holes in a incompressible substrate having films on either side thereof via a bonding layer; filling conductive paste into the through-holes; removing the films from the substrate; laminating metallic foils to either side of the substrate and heating same under pressures to harden the bonding layer, bonding the metallic foils to the substrate and electrically connecting the sides of the substrate to each other; and forming a circuit pattern by machining the metallic foils.

FIELD OF THE INVENTION

The present invention relates to a multi-layer circuit board with at least two or more layers of circuit patterns connected, and a method of manufacturing same.

BACKGOUND OF THE INVENTION

With a recent trend of electronic equipment becoming more miniaturized and higher in component density, there is an increasing demand for multi-layered circuit boards in the fields of both industrial and home-use equipment.

In the field of such multi-layer circuit board, there is a strong demand for the development of a method of making inner via-hole connection between multi-layered circuit patterns, and a circuit board having a highly reliable structure. As a method of manufacturing such circuit board, Japanese Laid-open Patent No. 6-268345 is proposing a method of manufacturing a high density circuit board having a novel structure wherein inner via-hole connections are made by conductive paste. This conventional method of manufacturing a circuit board will be described in the following.

A method of manufacturing a circuit board having four layers as a prior art multi-layer circuit board is described in the following.

First, a method of manufacturing a double-side circuit board being the base for a multi-layer circuit board is described.

FIG. 7 is a sectional view of the process for a conventional method of manufacturing double-side circuit boards for inner layers. In FIG. 7, the substrate is formed of prepreg sheet 21. The prepreg sheet 21 has a shape of 250 mm sq. and about 150 μm thick. For example, the prepreg sheet 21 is formed of a composite material having non-woven fabric made from aromatic polyamide fiber and thermosetting epoxy resin impregnated to the non-woven fabric. Releasing films 22 a, 22 b have a plastic film and a Si type releasing agent applied to one side of the film, and the releasing films 22 a, 22 b are about 16 μm in thickness. For example, as plastic film, polyethylene terephthalate is employed.

A method of bonding the prepreg 21 and releasing film 22 a, 22 b together is disclosed in Japanese Laid-open Patent No. 7-106760. Japanese Laid-open Patent No. 7-106760 refers to a method of continuously bonding releasing films 22 a, 22 b by dissolving the resin component of prepreg 21 with the use of a laminating apparatus.

A through-hole 23 is formed in the pregreg sheet 21 and releasing films 22 a, 22 b bonded together. The through-hole 23 is filled with conductive paste 24. Metallic foils 25 a, 25 b such as copper of 18 μm thick are bonded to either side of the prepreg sheet 21. The conductive paste 24 is electrically connected to the metallic foil 25 a, 25 b.

In FIG. 7,(a) the releasing films 22 a, 22 b are bonded to either side of the prepreg sheet 21. Next, (b) the through-holes 23 are formed, at predetermined portions, in the prepreg sheet 21 with the releasing films 22 a, 22 b bonded thereto, by using a laser beam machining process.

Next, (c) the conductive paste 24 is filled into the through-holes 23. In the method of filling the conductive paste 24, the prepreg sheet 21 having the through-holes 23 is placed on the table of a printer (not shown) and the conductive paste 24 is directly printed on the releasing film 22 a. In this case, the releasing films 22 a, 22 b serve a function as a printing mask and also a function to prevent contamination of the prepreg sheet 21.

Subsequently, (d) the releasing films 22 a, 22 b are removed from either side of the prepreg sheet 21.

Next, (e) the metallic foils 25 a, 25 b are laminated to either side of the prepreg sheet 21. And the laminated metallic foils 25 a, 25 b and prepreg sheet 21 are heated under pressures at a temperature of about 200° C. and pressure of about 4 MPa for one hour in a vacuum.

In this way, (f) the prepreg sheet 21 is compressed and becomes reduced in thickness (t2) to approximately 100 μm. At the same time, the prepreg sheet 21 and metallic foils 25 a, 25 b are bonded to each other. Further, the metallic foil 25 a disposed on the surface side and the metallic foil 25 b disposed on the back side are electrically connected to each other by the conductive paste 24 filled in the through-holes 23 formed at the predetermined positions.

After that, the metallic foils 25 a, 25 b are selectively etched, and then circuit patterns 31 a, 31 b are formed respectively on either side thereof. Thus, a double-side circuit board can be obtained.

FIG. 8 is a sectional view of the process for a conventional method of manufacturing a multi-layer circuit board, and the multi-layer circuit board is a four-layer circuit board.

In FIG. 8(a), a double-side circuit board 40 having circuit patterns 31 a, 31 b manufactured by the steps from (a) to (g) of FIG. 4 and prepreg sheets 21 a, 21 b with conductive paste 24 filled in through-holes 23 manufactured by the steps from (a) to (d) of FIG. 7 are prepared.

Next, as shown in FIG. 8(b), metallic foil 25 b, prepreg sheet 21 b, inner layer double-side circuit board 40, prepreg sheet 21 a, and metallic foil 25 a are positioned and laminated in this order.

Subsequently, the laminated board formed of these is heated under pressures at a temperature of about 200° C. and pressure of about 4 MPa for one hour in a vacuum, thereby curing the prepreg sheets 21 a, 21 b. Thus, as shown in FIG. 8(c), the prepreg sheets 21 a, 21 b are compressed and become reduced in thickness (t2) to 100 μm, then the double-side circuit board 40 and metallic foils 25 a, 25 b are bonded to each other. The circuit pattern 31 a and circuit pattern 31 b of the double-side circuit board 40 are connected to the metallic foils 25 a, 25 b by inner via-holes filled with conductive paste 24. Next, as shown in FIG. 8(d), the metallic foils 25 a, 25 b are selectively etched, thereby forming the circuit patterns 32 a, 32 b. In this way, a circuit board having four layers can be obtained.

However, in the above conventional method of manufacturing a circuit board, a prepreg sheet formed of half-cured resin are heated under pressures to bond the prepreg sheet to a metallic foil and also to harden the prepreg sheet. In this case, during heating under pressures, a cushion is generally used as an intermediate material to compensate for variation in thickness of the prepreg sheet. However, due to variation in quantity and fluidity of the resin contained in the prepreg sheet, the peripheral resin excessively flows during heating under pressures, resulting in insufficient application of the pressure to the central portion of the substrate, and accordingly, the conductive paste will not be sufficiently compressed. As a result, there has been a problem of fluctuation in connection resistance.

SUMMARY OF THE INVENTION

A multi-layer circuit board of the present invention comprises:

(a) a double-side circuit board including a first substrate, a plurality of first through conductors disposed in the first substrate, bonding layers disposed on either side of the first substrate, and a first circuit pattern and a second circuit pattern disposed on the surface of the bonding layer,

wherein the first circuit pattern and the second circuit pattern are electrically connected to each other by the first through conductors;

(b) an intermediate substrate including a second substrate, a plurality of second through conductors disposed in the second substrate, and a first bonding layer and a second bonding layer disposed on either side of the second substrate,

wherein the first bonding layer and the second bonding layer are disposed on either side of the substrate except the second through conductors, and

the intermediate substrate is disposed on the surface of at least one of the first circuit pattern and the second circuit pattern.

By the above configuration, the through conductors disposed between the respective circuit patterns become uniform in connection resistance, and consequently, a circuit board having a stable connection resistance can be obtained.

A method of manufacturing a circuit board of the present invention comprises the steps of:

(a) manufacturing an intermediate substrate, including the steps of:

(i) forming a substrate having incompressibility;

(ii) forming bonding layers on either side of the substrate;

(iii) forming a plurality of through-holes in the substrate; and

(iv) filling conductive paste into the through-holes,

(b) manufacturing a double-side circuit board, including the steps of:

(v) laminating metallic foils to either side of the intermediate substrate and heating same under pressures, and

(vi) forming a first circuit pattern and a second circuit pattern by processing the metallic foils.

By the above configuration, the conductors electrically connecting the first circuit pattern and the second circuit pattern to each other become uniform in connection resistance, and consequently, a circuit board having a stable connection resistance can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the process showing a method of manufacturing a double-side circuit board used for a circuit board in one embodiment of the present invention.

FIG. 2 is a sectional view of the process showing a method of manufacturing the first multi-layer circuit board of circuit boards in one embodiment of the present invention.

FIG. 3(a) is a sectional view of a circuit board for resistance measurement.

FIG. 3(b) is a plan view of the circuit board of FIG. 3(a).

FIG. 3(c) is a graphical representation of characteristics showing the chain resistance values at the substrate positions of a double-side circuit board used for a circuit board in one embodiment of the present invention.

FIG. 4 is a graphical representation of characteristics showing the relationship between resin quantity and resistance of bonding layers used for a circuit board in one embodiment of the present invention.

FIG. 5 is a sectional view of the process showing a method of manufacturing the second multi-layer circuit board of circuit boards in an embodiment of the present invention.

FIG. 6 is a sectional view of the process showing a method of manufacturing the third multi-layer circuit board of circuit boards in an embodiment of the present invention.

FIG. 7 is a sectional view of the process showing a method of manufacturing a double-side circuit board used for a circuit board in a prior art example.

FIG. 8 is a sectional view of the process showing a method of manufacturing a multi-layer circuit board in a prior art example.

Description of the Reference Numerals  1 Substrate  2 Bonding layer  2a First bonding layer  2b Second bonding layer  3 Releasing film  4 Through-hole  5 Conductive paste  5a Through conductor  6a First metallic foil  6b Second metallic foil  6c Third metallic foil  6d Fourth metallic foil  7a First circuit pattern  7b Second circuit pattern  8a Third circuit pattern  8b Fourth circuit pattern  9 Smoothing layer 10 Intermediate substrate 10a First intermediate substrate 10b Second intermediate substrate 14 Non-through-hole 20 Double-side circuit board 20a First double-side circuit board 20b Second double-side circuit board 30a First bonding substrate 30b Second bonding substrate

DETAILED DESCRIPTION OF THE INVENTION

A multi-layer circuit board of the present invention comprises:

(a) a double-side circuit board including a first substrate, a plurality of first through conductors disposed in the f irst substrate, bonding layers disposed on either side of the first substrate, and a first circuit pattern and a second circuit pattern disposed on the surface of the bonding layer,

in which the first circuit pattern and the second circuit pattern are electrically connected to each other by the first through conductors;

(b) an intermediate substrate including a second substrate, a plurality of second through conductors disposed in the second substrate, and a f irst bonding layer and a second bondi ng layer disposed on either side of the second substrate,

in which the first bonding layer and the second bonding layer are disposed on either side of the substrate except the second through conductors, and the intermediate substrate is disposed on the surface of at least one of the first circuit pattern and the second circuit pattern.

Preferably, the bonding layer is connected to at least one of the first bonding layer and the second bonding layer, while the intermediate substrate and the double-side circuit board are connected to each other.

Preferably, the multi-layer circuit board of the present invention further comprises:

(c) an additional circuit pattern disposed on the surface of the intermediate substrate,

the additional circuit pattern being electrically connected to at least one of the first circuit pattern and the second circuit pattern by the second through conductors.

A method of manufacturing a circuit board of the present invention comprises the steps of:

(a) manufacturing an intermediate substrate, including the steps of:

(i) forming a substrate having incompressibility,

(ii) forming bonding layers on either side of the substrate,

(iii) forming a plurality of through-holes in the substrate, and

(iv) filling conductive paste into the through-holes; and

(b) manufacturing a double-side circuit board, including the steps of:

(v) laminating metallic foils to either side of the intermediate substrate and heating same under pressures, and

(vi) forming a first circuit pattern and a second circuit pattern by machining the metallic foils.

Preferably, the step of manufacturing the intermediate substrate comprises a step of filling the conductive paste into the through-holes so that the conductive paste is protruded from the surface of the bonding layer, thereby forming a protrusion of the conductive paste, in the step (v), the protrusion of the conductive paste is compressed by the metallic foils, and the metallic foils disposed on either side of the intermediate substrate are electrically connected to each other by the conductive paste in the through-holes.

By this method, resin streak is stabilized during heating under pressures. As a result, a circuit board having a stable connection resistance can be obtained.

A method of manufacturing a circuit board in one embodiment of the present invention comprises the steps of:

making through-holes in an incompressible substrate having film material on either side thereof via a bonding layer;

filling conductive paste into the through-holes;

peeling off the film material from the substrate;

laminating metallic foils to either side of the substrate and heating same under pressures to harden the bonding layer and to bond the metallic foils to the substrate, establishing an electrical connection between the sides of the substrate; and

forming a circuit pattern by machining the metallic foil.

Due to this method, only the bonding layer on the substrate is to be deformed during heating under pressures. Accordingly, resin streak is reduced at the periphery of the substrate, and thereby, the conductive paste is uniformly pressurized. As a result, a circuit board having stable connection resistance can be obtained.

Preferably, the substrate has incompressibility. Thus, as compared to a prepreg sheet formed of half-cured resin, the substrate is higher in strength and is less deformed due to pressures during resin curing of the bonding layer in the process of manufacturing a circuit board, and consequently, it is possible to improve the coincidence between land and via.

A method of manufacturing a circuit board in one embodiment of the present invention comprises the steps of:

alternately positioning and laminating a plurality of insulating substrates and circuit boards having through-holes therein in a manner such that the outermost layer is the insulating substrate;

disposing metallic foils on either side thereof;

heating the whole under pressures; and

forming a circuit pattern on the metallic foil.

By this method, the connection resistance will be stabilized, and a multi-layer circuit board improved in coincidence between land and via can be obtained.

Preferably, the substrate has incompressibility. The substrate is a composite material of woven or non-woven fabric and thermosetting resin, based on inorganic material or aromatic polyamide. The composite material is cured by heating under pressures. By this configuration, the substrate is improved in incompressibility. Further, by selecting a material having excellent laser workability, the circuit board is enhanced in density and in wiring storage capacity, and moreover, a circuit board having excellent physical and environmental characteristics can be obtained.

Preferably, a circuit board having two or more layers of circuit patterns is a circuit board with interlayer connection already provided. The amount of conductive paste protruded is controlled according to the thickness of releasing film or film material. In this way, even when the substrate is changed in thickness, the conductive paste is uniformly compressed. As a result, stable connection resistance can be obtained.

Preferably, resin streak on the bonding layer is reduced. The bonding layer is in a range of about 5 μm to about 15 μm in thickness. Thus, the variation in resistance of the through-holes will be lessened.

In a circuit board having no smoothing layer in circuit pattern concavities of a circuit board having at least two layers of circuit patterns, the bonding layers on the surface and back sides are different in thickness, and only the bonding layer adhering to the circuit pattern concavities is thicker. That is, the bonding layers disposed on either side of the substrate are different in thickness. By this configuration, resin streak during lamination can be minimized. As a result, the connection resistance can be stabilized.

Preferably, the step of forming the substrate having incompressibility further includes a step of roughening the surface of the substrate before forming the bonding layer. That is, the surface of the substrate having incompressibility is roughened before the bonding layer is formed. Thus, the bonding strength between the substrate and the bonding layer will be enhanced due to anchor effect.

Preferably, the step of forming the substrate having incompressibility further includes a step of eliminating the residual stresses. That is, the substrate is used after eliminating the residual stresses generated during the manufacture of the incompressible substrate. As a result, the incompressible substrate becomes free from strain, and further, the coincidence between land and via can be improved.

The step of forming the substrate having incompressibility further includes a step of heat-treating the substrate at the glass transition temperature of the resin contained in the substrate. In this way, the residual stresses can be eliminated from the substrate.

Preferably, a method of manufacturing a circuit board further comprises:

a step of disposing a smoothing layer on the bonding layer in the circuit pattern concavities of at least one circuit pattern of the first circuit pattern and the second circuit pattern of the double-side circuit board. That is, the smoothing layer is formed on the bonding layer where no circuit pattern is formed. Thus, the resin to fill up the concavities where no circuit pattern is formed becomes unnecessary. Accordingly, the quantity of resin in the bonding layer may be reduced, and also, resin streak generated at the periphery of the substrate during heating under pressures will be reduced. As a result, stable connection resistance can be obtained.

Preferably, the smoothing layer is formed of cured resin or half-cured resin.

An exemplary embodiment of the present invention will be described in the following with reference to FIG. 1 to FIG. 6.

EXEMPLARY EMBODIMENT 1

The first exemplary embodiment of the present invention for a method of manufacturing a circuit board is described here.

FIG. 1 is a sectional view of the process for a method of manufacturing a double-side circuit board used for the circuit board in one embodiment of the present invention. FIG. 2 is a sectional view of the process for a method of manufacturing a multi-layer circuit board used for the circuit board in one embodiment of the present invention, showing an example of 4-layer circuit board.

First, a method of manufacturing a double-side circuit board used for the multi-layer circuit board will be described.

In FIG. 1, substrate 1 is 250 mm square and about 80 μm in thickness, having incompressibility. The substrate 1 is formed of composite material having non-woven fabric of aromatic polyamide fiber, and thermosetting epoxy resin impregnated to the non-woven fabric.

The first method of forming the substrate 1 is such that the composite material is heated under pressures while sandwiching both sides of the composite material with metallic foils. In this way, the thermosetting epoxy resin hardens, and the metallic foils adhere to the substrate. After that, the metallic foils are removed by etching.

Preferably, in the first method, the surfaces of the metallic foils sandwiching the composite material are previously roughened. Thus, roughened surfaces are formed on either side of the substrate 1 after removal of the metallic foils by etching. Accordingly, as stated later, the bonding strength between the substrate 1 and the bonding layer 2 may be enhanced.

The second method of forming the substrate 1 is such that the composite material is heated under pressures while sandwiching both sides of the composite material with heat resisting films. After that, the heat resisting films are removed. As a heat resisting film, for example, Teflon or polyamide film is employed.

Releasing films 3 are bonded via bonding layer 2 to either side of the substrate 1 thus formed. That is, in FIG. 1(a), releasing films 3 are bonded to either side of the substrate 1 by the bonding layers 2.

The bonding layer 2 is, for example, thermosetting epoxy resin in the B stage, and about 1 μm in thickness.

The releasing film 3 includes plastic film, and Si type releasing agent applied to one side of the film, and is about 16 μm in thickness. As the plastic film, for example, polyethylene phthalate is employed.

The releasing film 3 is 16 μin thickness, but the thickness of the releasing film 3 is changed according to the thickness of the substrate 1.

As the first method of disposing the bonding layer 2, the bonding layer 2 is disposed on the surface of the releasing film 3. For example, a bonding agent having epoxy resin diluted to a specified viscosity level with a solvent such as methyl ethyl ketone is applied to the releasing surface of the releasing film 3 by means of a roll coater, and the applied bonding agent is dried. In this way, the bonding layer 2 is disposed on the surface of the releasing film 3.

Also, as another method of disposing the bonding layer 2, the bonding layers 2 are disposed on either side of the substrate 1. For example, a bonding agent containing epoxy resin diluted to a specified viscosity level with a solvent such as methyl ethyl ketone is applied in a specific thickness to both sides of the substrate 1 by means of a roll coater or dipping method, and the applied bonding agent is dried. In this way, the bonding layers 2 are disposed on either side of the substrate 1.

In each of the above methods, the incompressible substrate 1 and the releasing film 3 are bonded to each other by melting the bonding layer 2 with use of a laminating apparatus.

In FIG. 1(b), through-hole 4 is formed in the substrate 1 and releasing film 3 bonded to each other. Preferably, a plurality of through-holes 4 are formed at the specified positions. That is, the through-holes 4 are formed by laser beam machining or the like in the predetermined portions of the substrate 1 with releasing films 3 bonded to either side thereof by the bonding layers 2. Or, the through-holes 4 are formed by drilling or the like.

Next, in FIG. 1(c), conductive paste 5 is filled into the through-holes 4. For example, the substrate 1 having through-holes 4 is placed on a table of a printer (not shown), and the conductive paste 5 is directly printed on the releasing film 3. In this case, the releasing film 3 serves a function as a printing mask and also serves a function to prevent contamination of the substrate 1.

The conductive paste 5 contains a conductive filler and resin. For example, as a conductive filler, copper powder of 2 μm in average grain size is used, and as resin, thermosetting epoxy resin (non-solvent type) and hardening agent are used. As the hardening agent, it is preferable to use an acid anhydride type hardening agent. For example, the conductive paste 5 is formed by kneading 85% by weight of copper powder, 12.5% by weight of thermosetting epoxy resin, and 2.5% by weight of acid anhydride type hardening agent by using three rolls.

Preferably, the through-holes 4 are a plurality of through-holes, and when the conductive paste 5 is filled in each of the through-holes 4, the releasing film 3 used has a thickness that is optional in accordance with the thickness of the substrate 1. Thus, the conductive past 5 can be uniformed in compressibility.

Next, as shown in FIG. 1(d), the releasing film 3 is peeled off from the substrate 1. After the releasing film 3 is removed, the conductive paste 5 equivalent in thickness to the releasing film 3 is formed in a state of protruding from the surface of bonding agent 2. In this way, intermediate substrate 10 is manufactured. That is, the intermediate substrate 10 includes the substrate 1, the bonding layers 2 disposed on either side of the substrate, and the conductive paste 5 filled in the through-holes.

Also, first metallic foil 6 a and second metallic foil 6 b are prepared. As such metallic foils, copper foils are used. The copper foil is about 300 mm square and 12 μm in thickness.

As shown in FIG. 1(e), the first metallic foil 6 a and the second metallic foil 6 b are laminated to either side of the substrate 1, and are subsequently heated under pressures at a temperature of about 200° C. and a pressure of about 4 MPa for one hour in a vacuum or inert gas. Thus, the bonding layer 2 hardens, and the substrate 1 is bonded to the metallic foils 6 a, 6 b. At the same time, the protruding conductive paste 5 is compressed, and the conductive paste is hardened to form through conductor 5 a. In this way, the first metallic foil 6 a and the second metallic foil 6 b disposed on either side of the substrate 1 are electrically connected to each other by the through conductor 5 a formed in the through-hole 4.

Next, as shown in FIG. 1(f), the first metallic foil 6 a is selectively etched to form a first circuit pattern 7 a. Similarly, the second metallic foil 6 b is selectively etched to form a second circuit pattern 7 b. In this way, a double-side circuit board 20 is formed.

Also, a method of manufacturing a multi-layer circuit board using a double-side circuit board thus formed is described with reference to FIG. 2. An example of a four-layer circuit board having four layers of circuit patterns will be described in the following.

FIG. 2 is a sectional view of the process showing a method of manufacturing a multi-layer circuit board.

As shown in FIG. 2(a), the double-side circuit board 20, first intermediate substrate 10 a, and second intermediate substrate 10 b are prepared.

The double-side circuit board 20 is manufactured by a process shown in FIG. 1 described above. The first circuit pattern 7 a is disposed, by the bonding layer 2, on the first surface side of the substrate 1. The second circuit pattern 7 b is disposed, by the bonding layer 2, on the second surface side of the substrate 1. The first circuit pattern 7 a and the second circuit pattern 7 b are electrically connected to each other by the through conductor 5 a disposed in the through-hole.

The first intermediate substrate 10 a and the second intermediate substrate 10 b are prepared. These intermediate substrates 10 a, 10 b are nearly same in configuration as the intermediate substrate 10 shown in FIG. 1(d) described above. That is, each of the first intermediate substrate 10 a and the second intermediate substrate 10 b includes the substrate 1, first bonding layer 2 a and second bonding layer 2 b disposed on either side of the substrate, and the conductive paste 5 disposed in the through-hole. However, the first bonding layer 2 a and the second bonding layer 2 b are different in thickness. The conductive paste 5 are protruding from the surfaces of the respective bonding layers 2 a, 2 b. Each of the bonding layers 2 a, 2 b is resin in the B stage, and the thickness of each bonding layer is preferable to be in a range from about 10 μm to about 15 μm. Each bonding layer contains thermosetting resin such as epoxy resin. The B stage is a stage of being half-cured of thermosetting resin.

In FIG. 2(a), the first circuit pattern 7 a and the second circuit pattern 7 b are 12 μm in thickness. The first bonding layer 2 a and the second bonding layer 2 b are respectively about 15 μm and 10 μm in thickness.

Next, as shown in FIG. 2(b), fourth metallic foil 6 d, second intermediate substrate 10 b, double-side circuit board 20, first intermediate substrate 10 a, and third metallic foil 6 c are positioned and laminated in this order.

The third metallic foil 6 c is placed on the second bonding layer 2 b side of first intermediate substrate 10 a, while the first bonding layer 2 a side of first intermediate substrate 10 a is placed on the first circuit pattern 7 a side of double-side circuit board 20. The fourth metallic foil 6 d is placed on the second bonding layer 2 b side of second intermediate substrate 10 b, while the first bonding layer 2 a side of second intermediate substrate 10 b is placed on the second circuit pattern 7 b side of double-side circuit board 20. That is, the first bonding layer 2 a side being greater in thickness is laminated onto the double-side circuit board 20, while the second bonding layer 2 b side being less in thickness is laminated onto the metallic foil 6 c, 6 d. Since the first bonding layer 2 a side having a greater thickness is laminated to the double-side circuit board 20, the first bonding layers 2 a may fill up the concavities of the first circuit pattern 7 a and the second circuit pattern 7 b.

Next, as shown in FIG. 2(c), these laminated members are heated under pressures at a temperature of about 200° C. and a pressure of about 4 MPa for one hour in a vacuum or inert gas. In this way, the conductive paste 5 protruding from the surface of bonding layer 2 a, 2 b is compressed and hardened, thereby forming through conductor 5 a. At the same time, each of the bonding layers 2 a, 2 b is hardened. Thus, the double-side circuit board 20 and the metallic foils 6 c, 6 d are bonded to each other, and also, the circuit patterns 7 a, 7 b establish inner via-hole connections with the respective metallic foils 6 c, 6 d via the through conductor 5 a. Further, a laminated structure is manufactured in a state such that the concavities of the first circuit pattern 7 a and the second circuit pattern 7 b are filled up with the first bonding layers 2 a.

Also, as shown in FIG. 2(d), the metallic foils 6 c, 6 d disposed on either side of the laminated body are selectively etched to form the third circuit pattern 8 a and the fourth circuit pattern 8 b. In this way, a multi-layer circuit board 30 having four layers of circuit patterns is obtained.

Instead of the double-side circuit board, by using the multi-layer circuit board 30 manufactured as described above and also a required number of intermediate substrates, and by laminating the multi-layer circuit board 30 and the intermediate substrates in the same way as described above, it is possible to obtain a multi-layer circuit board having four or more layers of circuit patterns.

FIG. 3(a) is a sectional view of a circuit board for measuring the resistance values of the circuit board. FIG. 3 (b) shows a plan view of same. FIG. 3(c) shows the chain resistance (connection resistance) at the substrate positions of a double-side circuit board obtained by the present exemplary embodiment mentioned above and of a double-side circuit board obtained by a conventional manufacturing method.

The prior art manufacturing method is, as shown in FIG. 7, a method of forming circuit patterns on a substrate by using a prepreg sheet.

As shown in FIG. 3(c), the resistance of a conventional circuit board manufactured by using a prepreg sheet having much resin is higher at the central portion of the substrate and lower at the end portion of the substrate. The substrate at the central portion of the conventional circuit board is about 10 μm thicker as compared with the end portion. Accordingly, when the prepreg sheet is heated under pressures, the resin streak becomes uneven at the end portion of the substrate or the prepreg itself is compressed, and the pressures are not evenly applied to the substrate. As a result, the resistance varies with the positions of the circuit board.

On the other hand, the resistance of the circuit board in the present embodiment using an incompressible substrate is identical at all positions of the circuit board. In addition, the circuit board of the present embodiment is identical in thickness at all positions thereof.

Next, a variety of circuit boards having various bonding layers different in thickness were manufactured, and the contact resistance of each circuit board was measured. The results of the measurements are shown in FIG. 4. That is, it has been confirmed that the quantity of resin on the bonding layer is closely related to the resistance. The connection resistance values were measured with respect to circuit boards having bonding layers of 5 μm, 10 μm, 15 μm, 20 μm, and 30 μm in thickness.

As shown in FIG. 4(a), FIGS. 4(b) and FIG. 4(c), the circuit board having a bonding layer of 5 μm, 10 μm or 15 μm thick is identical in resistance at all positions of the circuit board. That is, all the positions of the circuit board are uniform in connection resistance.

On the other hand, as shown in FIG. 4(e), the circuit board having a bonding layer of 30 μm thick is higher in resistance at the central position rather than at the end position of the circuit board. That is, the connection resistance varies with the positions of the substrate.

Also, the resistance at the central position of the substrate becomes higher with increase in thickness of the bonding layer, causing the difference in thickness to become greater between the central position and the end position of the substrate.

From these experimental results, it is preferable to use bonding layers less in thickness because of being uniform in connection resistance. That is, the thickness of a bonding layer is preferable to be ranging from about 5 μm to about 15 μm. In case the bonding layer is thinner than about 5 μm, it may sometimes result in reduction of the adhesion between the substrate 1 and the circuit pattern 6 a, 6 b.

Also, by using a multi-layer circuit board having four layers of circuit patterns using the double-side circuit boards, the connection resistance of the circuit board was measured in the same way as described above. As a result, the same as in the measurement of the above double-side circuit board, the connection resistance was uniform when the thickness of the bonding layer is in a range from about 5 μm to about 15 μm. Further, as the bonding layer increased in thickness, the resistance at the central position of the substrate became higher, causing the difference in thickness to become greater between the central position and the end position of the substrate.

In the present exemplary embodiment, the metallic foils 6 a, 6 b of the double-side circuit board are 12 μm in thickness. However, when a multi-layer circuit board is manufactured by using metallic foils thicker than 12 μm, it is preferable that the concavities of circuit patterns 7 a, 7 b be filled up with resin or the like in advance. That is, the surfaces can be flattened by filling up the concavities of circuit patterns 7 a, 7 b. In this way, the first bonding layer 2 a of intermediate substrate 10 a, 10 b can be reduced in thickness. It is because the first bonding layer 2 a that functions to fill up the concavities of circuit patterns 7 a, 7 b requires less quantity of resin. Accordingly, by using the first bonding layer 2 a reduced in thickness, it is possible to make the connection resistance identical at all positions of the multi-layer circuit board.

The substrate used in the present embodiment is characterized in that the substrate has incompressibility. Therefore, it is possible to greatly reduce the dimensional strain generated in a step of pressure application for the manufacture of a circuit board. For example, the dimensional strain at the position of the multi-layer circuit board in the present embodiment is less than ⅕ of the strain in a conventional circuit board using a substrate having compressibility. As a result, the coincidence between via and land may be enhanced.

Preferably, the incompressible substrate contains resin. And, in the step of FIG. 1, preferably, the substrate is heated at a temperature higher than the glass transition temperature (Tg) of the resin contained. Thus, residual stresses generated during the manufacture of an incompressible substrate will be reduced. Accordingly, in the circuit board manufacturing process, the dimensional strain generated due to stresses will be reduced. Further, the coincidence between via and land is enhanced. As a result, the laminating accuracy between the double-side circuit board and intermediate substrate will be improved.

EXEMPLARY EMBODIMENT 2

The second exemplary embodiment of the present invention for a method of manufacturing a circuit board will be described in the following.

FIG. 5 is a sectional view of the process showing a method of manufacturing a multi-layer circuit board in the present exemplary embodiment, and the multi-layer circuit board has four layers of circuit patterns.

In FIG. 5(a), first double-side circuit board 20 a, second double-side circuit board 20 b, and insulating substrate 10 are prepared. These double-side circuit boards 20 a, 20 b are same in configuration as the double-side circuit board 20 manufactured in the first exemplary embodiment described above.

Intermediate substrate 10 is same in configuration as the intermediate substrate 10 manufactured in the first exemplary embodiment described above. That is, the intermediate substrate 10 comprises the incompressible substrate 1, bonding layers 2 a, 2 b disposed on either side of the substrate 1, and conductive paste 5 disposed in the through-hole. The conductive paste 5 is protruding from the through-hole. The bonding layers 2 a, 2 b are 10 μm thick and in a state of being half-cured at the B stage.

Next, as shown in FIG. 5(b), smoothing layer 9 is formed in concavities of second circuit pattern 7 b on each of the double-side circuit boards 20 a, 20 b. The surface of the smoothing layer 9 is nearly the same in height as the surface of the second circuit pattern 7 b.

The smoothing layer 9 is disposed in concavities by using a smoothing material. The smoothing material contains thermosetting resin and a solvent such as methyl ethyl ketone. The smoothing material is applied to concave portions by a squeezing process or the like, and is heated at a temperature of about 150° C for two minutes. In this process, the solvent contained is volatilized and the thermosetting resin is hardened to a state of being half-cured at the B stage. Thus, the smoothing layer 9 is disposed. The smoothing layer 9 in a state of being half-cured is nearly the same in thickness as the circuit patterns 7 a, 7 b. For example, epoxy resin is employed as the thermosetting resin.

The following method can also be used as a method of disposing the smoothing layer 9. That is, epoxy resin is applied thicker than the circuit patterns 7 a, 7 b. And the epoxy resin is heated and cured. After that, the cured epoxy resin is polished to expose the surfaces of the circuit patterns 7 a, 7 b. In this way, the smoothing layer 9 is formed in same height as that of circuit pattern 7 a, 7 b. The smoothing layer 9 is disposed on the second circuit pattern side at one side of each double circuit board 20 a, 20 b.

Next, as shown in FIG. 5(c), the second double-side circuit board 20 b, intermediate substrate 10, and first double-side circuit board 20 a are positioned and laminated in this order. In this case, the second circuit pattern 7 a having the smoothing layer 9 of each double-side circuit board 20 a, 20 b is laminated to the back of the intermediate substrate 10.

Also, as shown in FIG. 5(d), these laminated members are heated under pressures at a temperature of about 200° C. and a pressure of about 4 MPa for one hour in a vacuum or inert gas. Then, the bonding layer 2 a, 2 b of the intermediate substrate 10 and the smoothing layer 9 of the double-side circuit board 20 a, 20 b are cured. At the same time, the double-side circuit boards 20 a, 20 b are respectively laminated to the insulating substrate 10.

Further, the protruding conductive paste 5 is compressed, and simultaneously, the conductive paste in the through-hole is hardened to form through conductor 5 a. The circuit patterns 7 a, 7 b of the double-side circuit board 20 a establish inner via-hole connections with the circuit patterns 7 a, 7 b of the double-side circuit board 20 b via the through conductors 5 a. In this way, a multi-layer circuit board having four-layer circuit patterns can be obtained. By the bonding layers 2 a, 2 b disposed on either side of the intermediate substrate 10, the double-side circuit boards 20 a, 20 b are laminated to each other. Then, the circuit patterns 7 a, 7 b are electrically connected to each other by the through conductor 5 a.

Also, by using a plurality of double-side circuit boards 20 and a plurality of intermediate substrates 10, a multi-layer circuit board having a plurality of circuit patterns may be manufactured by the same method as described above. In this case, a plurality of double-side circuit boards with intermediate substrates 10 sandwiched therebetween are laminated so that the double-side circuit board is formed as the outermost layer. Then, smoothing layer 7 is formed on the surface of double-side circuit board placed on the intermediate substrate. For example, a smoothing layer is formed on the inner side of the double-side circuit board located at the outermost position, and thereby, the smoothing layers are disposed on either side of the internally located double-side circuit board. In this way, a multi-layer circuit board having the desired number of circuit patterns may be manufactured.

The connection resistance was measured the same as in the first exemplary embodiment with respect to the multi-layer circuit board obtained in the present exemplary embodiment. Consequently, it has been confirmed that the connection resistance is identical at all positions of the circuit board. That is, stable and uniform contact resistance may be obtained at all positions of the circuit board. In addition, it has been confirmed that the coincidence between via and land is enhanced.

EXEMPLARY EMBODIMENT 3

The third exemplary embodiment of the present invention for a method of manufacturing a circuit board will be described in the following.

FIG. 6 is a sectional view of the process showing a method of manufacturing a multi-layer circuit board in an exemplary embodiment, and the multi-layer circuit board has four-layer circuit patterns.

As shown in FIG. 6(a), double-side circuit board 20, first bonding substrate 30 a, and second bonding substrate 30 b are prepared. As the double-side circuit board 20, a double-side circuit board 20 same as the double-side circuit board 20 manufactured in the first exemplary embodiment is employed. The double-side circuit board 20 comprises substrate 1, bonding layers 2 disposed on either side of the substrate 1, first circuit pattern 7 a and second circuit pattern 7 b bonded thereto by the bonding layer 2. Each of the circuit patterns 7 a, 7 b is about 12 μm in thickness. The first circuit pattern 7 a and the second circuit pattern 7 b are electrically connected to each other by through conductor 5 a. The bonding substrate 30 includes the incompressible substrate 1 and the bonding layers 2 a, 2 b disposed on either side of the substrate 1. The bonding substrates 30 a, 30 b serve a function as an insulating material. Each of the bonding layers 2 a, 2 b contains epoxy resin of about 15 μm thick in the B stage.

Next, as shown in FIG. 6(b), the first bonding substrate 30 a and the second bonding substrate 30 b are generally positioned and laminated onto either side of the double-side circuit board 20. After that, releasing film 3 is disposed on each outermost side of the bonding substrates 30 a, 30 b. By heating to melt the bonding layers 2 a, 2 b, the releasing films 3 are bonded to the respective bonding materials. In that case, the bonding layers 2 a, 2 b of the bonding substrates 30 a, 30 b are kept in the B stage.

Also, as shown in FIG. 6(c), non-through-hole 14 is formed by using a laser beam machining process in recognition of circuit patterns 7 a, 7 b, and via so that the through-hole coincides with the circuit pattern 7 a, 7 b. The non-through-hole 14 is reaching the surface of the circuit pattern 7 a, 7 b. At the bottom of non-through-hole 14, the resin content of the bonding layer 2 a is eliminated by laser, and the metal surface of the circuit pattern 7 a, 7 b is exposed at the bottom of non-through-hole 14. As the laser used for making holes, carbon dioxide gas laser or the like is employed since such laser is reverberated by the metal of the circuit pattern 7 a, 7 b.

After that, as shown in FIG. 6(d), conductive paste 5 is filled into the non-through-hole 14, using the releasing film 3 as a mask.

Next, as shown in FIG. 6(e), the releasing film 3 is removed from each outermost surface of the laminated body. With the releasing film 3 removed, the conductive paste 5 equivalent in thickness to the releasing film 3 is protruded from the outermost bonding layer 2 a .

And, as shown in FIG. 6(f), metallic foils 6 a, 6 b of 12 μm thick are placed on either side of the laminated body. For example, copper is used as the metallic foil 6 a, 6 b. After that, the laminated members are heated under pressures at a temperature of about 200° C. and a pressure of about 4 MPa for one hour in a vacuum or inert gas.

Thus, as shown in FIG. 6(g), the bonding layers 2 a, 2 b of the respective bonding substrates 30 a, 30 b are cured, then the bonding substrates 30 a, 30 b, double-side circuit board 20, and respective metallic foils 6 a, 6 b are bonded to each other. At the same time, the protruding conductive paste 5 is compressed against the conductive paste in the through-hole and is hardened to form through conductor 5 a. As a result, the metallic foils 6 a, 6 b establish inner via-hole connections with the respective circuit patterns 7 a, 7 b via the through conductor 5 a formed in the non-through-hole 14.

Next, as shown in FIG. 6(h), the metallic foils 6 a, 6 b are selectively etched to form additional circuit patterns 8 a, 8 b. In this way, a circuit board having four-layer circuit patterns is obtained.

The multi-layer circuit board thus manufactured is identical in connection resistance at all positions thereof, same as in the first exemplary embodiment, and very stable in connection resistance. Further, the coincidence between via and land is enhanced.

Instead of the above double-side circuit board 20, a muti-layer circuit board having six-layer circuit patterns may be obtained, using the above four-layer circuit board, by performing the same process as described above. Similarly, instead of the double circuit board 20, a multi-layer circuit board having eight-layer circuit patterns may be obtained, using the above six-layer circuit board, by performing the same process as described above.

Thus, a circuit board having four or more layers of circuit patterns may be obtained by using the method of the present exemplary embodiment.

In the first, second and third exemplary embodiments described above, a material ensuring electrical insulation is used as an incompressible substrate. For example, as the incompressible substrate, a mixture of resin and filler or a ceramic material is employed. As the filler, organic or inorganic material powder, or fiber is employed.

Preferably, the substrate includes fiber aggregate, and thermosetting resin impregnated to the fiber aggregate.

Further preferably, the fiber aggregate used is non-woven fabric or woven fabric of fiber such as aromatic polyamide fiber, ceramic fiber, and glass fiber. These materials as substrates are already compressed and formed. That is, when the incompressible substrate is subjected to pressures, the substrate remains unchanged in thickness without being compressed.

For example, a composite material of non-woven fabric of aromatic polyamide fiber and thermosetting epoxy resin impregnated to the non-woven fabric or a composite material of glass fiber fabric and thermosetting epoxy resin impregnated to the fabric is used as a preferable material.

For example, such composite material of glass fabric and epoxy resin sandwiched between metallic foils was heated under pressures. After that, the composite material thus formed was etched to remove the metallic foils, thereby forming a substrate. By using the substrate, a multi-layer circuit board was manufactured in the same way as in the first exemplary embodiment described above. For the multi-layer circuit board, the connection resistance was measured the same as in the first exemplary embodiment. As a result, it has been confirmed that the same advantages as in the first exemplary embodiment can be obtained.

Also, in the manufacture of a double-side circuit board in the first, second, and third exemplary embodiments, it is possible to use a double-side board in which through-holes are made by drilling and the through conductor is, for example, a metal deposited by electroplating. By using such double-side circuit board, it is possible to obtain a circuit board having smoother surfaces. Also in this case, the advantages obtained are same as in the first, second and third exemplary embodiments.

By the configuration as described above, stable connection resistance can be obtained at all positions of the circuit board. Further, the laminating accuracy of the circuit patterns is improved and, in addition, the coincidence between through conductor and circuit pattern is enhanced. As a result, a circuit board having excellent electrical characteristics may be obtained. 

What is claimed is:
 1. A multi-layer circuit board, comprising: (a) a double-side circuit board including a first substrate, a plurality of first through conductors disposed in said first substrate, bonding layers disposed on either side of said first substrate, and a first circuit pattern and a second circuit pattern disposed on the surface of said bonding layer, said first circuit pattern and said second circuit pattern being electrically connected to each other by said first through conductors; (b) an intermediate substrate including a second substrate, a plurality of second through conductors disposed in said second substrate, and a first bonding layer and a second bonding layer disposed on either side of said second substrate, said first bonding layer and said second bonding layer being disposed on either side of said substrate except said second through conductors, and said intermediate substrate being disposed on the surface of at least one of said first circuit pattern and said second circuit pattern.
 2. The multi-layer circuit board of claim 1, wherein said bonding layer is connected to at least one of said first bonding layer and said second bonding layer, while said intermediate substrate and said double-side circuit board are connected to each other.
 3. The multi-layer circuit board of claim 1, further comprising: (c) an additional circuit pattern disposed on the surface of said intermediate substrate, said additional circuit pattern being electrically connected to at least one of said first circuit pattern and said second circuit pattern by said second through conductors.
 4. The multi-layer circuit board of claim 3, wherein said intermediate substrate includes a first intermediate substrate disposed on the surface of said first circuit pattern, and a second intermediate substrate disposed on the surface of said second circuit pattern; said additional circuit pattern includes a third circuit pattern disposed on the surface of said first intermediate substrate, and a fourth intermediate substrate disposed on the surface of said second intermediate substrate; said third circuit pattern is electrically connected to said first circuit pattern by the second through conductors disposed in said first intermediate substrate; and said fourth circuit pattern is electrically connected to said second circuit pattern by the second through conductors disposed in said second intermediate substrate.
 5. The multi-layer circuit board of claim 3, wherein said additional circuit pattern is a circuit pattern located at the outermost layer position.
 6. The multi-layer circuit board of claim 1, wherein said double-side circuit board further includes a smoothing layer, and said smoothing layer is disposed on said bonding layer in concavities where none of said first circuit pattern and said second circuit pattern are formed.
 7. The multi-layer circuit board of claim 1, wherein said first substrate has incompressibility.
 8. The multi-layer circuit board of claim 1, wherein said second substrate has incompressibility.
 9. The multi-layer circuit board of claim 1, wherein said first substrate and said second substrate have incompressibility that is given by heating a composite material under pressures, and said composite material contains woven or non-woven fabric using aromatic polyamide as main material, and thermosetting resin.
 10. The multi-layer circuit board of claim 1, wherein said first bonding layer and said second bonding layer are formed in a state of being half-cured, and become cured after lamination of said double-side circuit board and said intermediate substrate.
 11. The multi-layer circuit board of claim 1, wherein said double-side circuit board includes a plurality of double-side circuit boards; said intermediate substrate is located between said double-side circuit boards; said bonding layer is bonded to at least one of said first bonding layer and said second bonding layer, while said intermediate substrate and said double-side substrate are bonded to each other; and said circuit patterns of said respective double-side circuit boards are electrically connected to each other by said second through conductors.
 12. The multi-layer circuit board of claim 1, wherein said second bonding layer disposed on said intermediate substrate is positioned so as to be filled into concavities of at least one circuit pattern of said first circuit pattern and said second circuit pattern, on a surface where said double-side circuit board and said intermediate substrate are laminated.
 13. The multi-layer circuit board of claim 1, wherein said first bonding layer and said second bonding layer contain thermosetting resin.
 14. The multi-layer circuit board of claim 1, wherein at least one of said first substrate and said second substrate includes a roughened surface, and said bonding layer, said first bonding layer, and said second bonding layer are disposed on the roughened surface.
 15. The multi-layer circuit board of claim 1, wherein said bonding layer is in a range of about 5 μm to about 15 μm in thickness. 